Most fail safe brakes are actuated by heavy mechanical springs, such as powerful compression springs or stacked Belville washers. These springs bear against a piston, holding frictional material against the rotative member of the brake to provide the stopping or holding torque to hold the brake in the on position. High hydraulic pressure is used on the front of the piston to force it back against the spring force to keep the brake released. There are disadvantages to this system.
For example, an external high pressure hydraulic source must be provided and applied to the brake. Adjusting the brake and pressures in some cases means disassembly of the unit to change the stack of Belville washers or changing the compression spring for a higher or lower spring force. Wear of the frictional material changes the compression height of the springs, resulting in reduced braking forces; and in some cases a manual screw adjuster must be used to maintain the spring force. Also, releasing this type of brake where there is no hydraulic pressure available in a brake-down situation usually means having to use some external mechanical force to compress the spring.
It is the object of the present invention to provide a novel fail safe unit not dependent on external hydraulics or force springs, and wherein only low pressure air is required; and simply by the use of a three way valve, the brake can be applied or retracted at any time desired; braking torques can be varied simply by regulating the input air pressure; the fail safe pressures are not greatly affected by frictional material wear; and break-down brake release is achieved by releasing tank pressure by means of a relief valve without further mechanism or action required.